1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to an electroluminescence (EL) device made using a nanotip electrode, with a nanotip-contoured phosphor layer.
2. Description of the Related Art
The generation of light from semiconductor devices is possible, regardless of whether the semiconductor material forms a direct or indirect bandgap. High field reverse biased p-n junctions create large hot carrier populations that recombine with the release of photons. For silicon devices, the light generation efficiency is known to be poor and the photon energy is predominantly around 2 eV. The conversion of electrical energy to optical photonic energy is called electroluminescence (EL). Efficient EL devices have been made that can operate with small electrical signals, at room temperature. However, these devices are fabricated on materials that are typically not compatible with silicon, for example type III-V materials such as InGaN, AlGaAs, GaAsP, GaN, and GaP. An EL device built on one of these substrates can efficiently emit light in a narrow bandwidth within the visible region, depending on the specific material used. Additionally, type II-VI materials such as ZnSe have been used. Other type II-VI materials such as ZnS and ZnO are known to exhibit electroluminescence under ac bias conditions. These devices can be deposited onto silicon for use in light generating devices if special (non-conventional) CMOS processes are performed. Other classes of light emitting materials are organic light emitting diodes (OLEDs), nanocrystalline silicon (nc-Si), and polymer LEDs.
Silicon has conventionally been considered unsuitable for optoelectronic applications, due to the indirect nature of its energy band gap. Bulk silicon is indeed a highly inefficient light emitter. Among the different approaches developed to overcome this problem, quantum confinement in Si nanostructures and rare earth doping of crystalline silicon have received a great deal of attention.
A simple and efficient light-emitting device compatible with silicon would be desirable in applications where photonic devices (light emitting and light detecting) are necessary. Efficient silicon substrate EL devices would enable a faster and more reliable means of signal coupling, as compared with conventional metallization processes. Further, for intra-chip connections on large system-on-chip type of devices, the routing of signals by optical means is also desirable. For inter-chip communications, waveguides or direct optical coupling between separate silicon pieces would enable packaging without electrical contacts between chips. For miniature displays, a method for generating small point sources of visible light would enable simple, inexpensive displays to be formed.
FIG. 1 is a partial cross-sectional view of a thin-film, solid-state Si phosphor EL device (prior art). EL devices compatible with Si are currently being sought for a number of applications such as optical interconnects. An AC EL device may consist of a substrate 1, an optional bottom electrode 2, a phosphor layer 4, sandwiched by a top 5 and bottom 3 dielectric layer, and a top transparent electrode 6. Such a device typically requires high operating fields in order to inject electrons from states at the interface into the phosphor layer. The electrons are then, accelerated by the field, gaining energy until they radiatively decay at luminescent centers. As shown, all layers in these devices are planar.
Nanostructured materials such as nanowires, nanorods, and nanoparticles, have potential for use in applications such as nanowire chemical and bio sensors, nanowire LEDs, nanowire transistors, nanowire lasers, to name a few examples. Materials such as Si, Ge, other elemental semiconductors, ZnO, and other binary semiconductors have been made into nanostructures. One of the primary methods for nanowire formation is the vapor-liquid solid transport method with which a catalyst can be used to grow a nanowire from the gas phase. Other methods have also been used.
To aid in the generation of photoluminescence (PL), nanostructured electrodes have been used to develop higher intensity fields in Si compatible phosphor materials. For example, Hsu et al., in SILICON PHOSPHOR ELECTROLUMINESCENCE DEVICE WITH NANOTIP ELECTRODE, Ser. No. 11/061,946, filed on Feb. 17, 2005, assigned to the same assignee as the instant application, describes EL devices made using iridium oxide nanotips. It would be desirable if PL intensity could be further improved by continuing the development of EL nanostructures. However, the high surface area inherent to a nanostructures creates problems in conformally covering the nanotip surfaces. This conformality problem, in turns, can result in the formation of gaps and air pockets between the nanotip electrode and the overlying phosphor layer.
It would be advantageous if the intensity of a Si compatible phosphor EL device could be enhanced through the formation of nanostructures.
It would be advantageous if the Si phosphor layer of an EL device could be contoured to match the shape of a nanotip electrode.